Finer circuit patterns have grown in demand in response to finer processing of semiconductor devices in recent years, particularly in response to higher levels of integration of large scale integrated circuits. In order to achieve finer circuit patterns, a technique for making fine lines in a wiring pattern making up a circuit, a technique for making a fine pattern of a contact hole used for wiring between layers making up a cell, and other techniques are necessary. These kinds of patterning have been performed by optical lithography using photomasks, and thus techniques for forming finer photomask patterns with high precision have been demanded.
In order to form a photomask pattern on a photomask substrate with high precision, it is necessary to first form a resist pattern on a photomask blank with high precision. Actually a semiconductor substrate is processed by optical lithography using reduced projection, and thus the size of a pattern formed on a photomask used in this exposure process is generally about four times as large as the size of a pattern to be printed on the substrate.
However, the sizes of circuit patterns written by optical lithography have been much smaller than the wavelengths of exposure light in recent years. When a pattern simply four times as large as the shape of a written circuit pattern is used as a photomask pattern, it is difficult to transfer the shape of the photomask pattern as it is on a resist film due to the influence of interference or the like of light generated during exposure. For this reason, it has been necessary to form a photomask pattern with extremely high precision on a photomask serving as a master of a written circuit pattern. In order to reduce the influence of the interference or the like of light during exposure, a photomask pattern (so-called OPC pattern) having a more complicated shape than an actually written circuit pattern may be used in some cases.
As described above, also in lithography techniques for obtaining photomask patterns, high patterning accuracy is desired as in optical lithography techniques for performing fine processing on semiconductor substrates and so on. A limited resolution is generally used as an index of lithography performance. However, in reality, a lithography technique for a process of patterning a photomask requires a limited resolution equal to or higher than that of an optical lithography technique for a process of printing a pattern formed on the photomask onto a semiconductor substrate.
Photomask patterns have been mainly formed by electron beam exposure, though an exposing method using light has been available. Generally, a photoresist film is first formed on a photomask blank including a light shielding layer on a transparent substrate, a pattern is written on the photoresist film by an electron beam, and the photoresist film is developed, so that a resist pattern is obtained. After that, a pattern (photomask pattern) including a light shielding part and a translucent part is formed by using the resist pattern as an etching mask for a mask layer. An ordinary mask layer has a laminated structure of an antireflective layer and a light shielding layer.
As finer photomask patterns are formed, finer resist patterns are formed. When only a resist pattern is made finer without reducing the thickness of a resist film, a resist part acting as an etching mask for a mask layer increases in aspect ratio (a ratio between the thickness of the resist film and a pattern width). Generally, when the aspect ratio of a resist pattern increases, the shape of the pattern is apt to deteriorate, reducing the accuracy of transferring the pattern to a mask layer with the resist pattern serving as an etching mask. Moreover, in extreme cases, a part of the resist pattern may fall or come off, so that pattern loss may occur. Therefore, as finer photomask patterns are formed, it is necessary to reduce the thickness of a resist used as an etching mask for patterning a mask layer and prevent an extremely high aspect ratio.
Incidentally, a number of materials have been already proposed as light shielding film materials for performing patterning on a mask layer by using a resist as an etching mask. Of these materials, regarding etching of a chromium compound film, a large amount of information is available. In practical use, a chrome compound is always used as a light shielding film material, which has been substantially established as a standard processing process. For example, Patent document 1 (Japanese Patent Laid-Open No. 2003-195479), Patent document 2 (Japanese Patent Laid-Open No. 2003-195483), and Patent document 3 (Japanese Registered Utility Model No. 3093632) disclose examples of the configuration of a photomask blank in which a light shielding film having a light shielding property demanded of photomask blanks for ArF exposure is made of a chromium compound. The thickness of the light shielding film is set at about 50 nm to 77 nm, and the light shielding film having this thickness is patterned only with a resist mask.
A light shielding film of a chromium film and a chromium compound film is generally patterned by chlorine-based dry etching containing oxygen. In many cases, this etching condition produces an etching effect that is not negligible on an organic film such as a resist. Thus when etching the light shielding film with a relatively thin resist film serving as a mask, the resist is damaged and the shape of the resist pattern is changed during the etching, so that it becomes difficult to precisely transfer the original resist pattern onto the light shielding film.
However, it is technically difficult to allow a photoresist, which is an organic film, to have etching resistance (plasma resistance) enabling a high resolution and high patterning accuracy. Thus in order to reduce the load on a resist during the etching of a light shielding film and form a photomask pattern with higher precision, the selection of a material of the light shielding film has to be reexamined.
[Patent document 1] Japanese Patent Laid-Open No. 2003-195479
[Patent document 2] Japanese Patent Laid-Open No. 2003-195483
[Patent document 3] Japanese Registered Utility Model No.
[Patent document 4] Japanese Patent Laid-Open No. 2001-312043
[Patent document 5] Japanese Patent Laid-Open No. 63-85553